Power detection today is done with dedicated 20 dB couplers, not directly on the forward path and generally only provides an amplitude signal without any phase information. Output matching network tuning is usually attempted at the 50 Ohm point, forcing the use of high voltage compliant technologies.
To build power amplifiers (PA) with low sensitivity to load phase and angle changes, the use of quadrature power amplifiers is becoming more and more common. These amplifiers are constructed by building two amplifiers, splitting the input signal into a 0 and a 90 degree path and then recombining the output in a hybrid, i.e. a 3 dB directional coupler. The directional coupler will combine the output of both amplifiers as an in phase signal at the output port, while any power coming into the output port will be reflected by the power amplifiers into the reference port where it will see a 180 deg phase shift and therefore will be cancelled.
However current implementations of hybrids are very large in area, only work over a limited frequency range and do not provide a feedback signal for the forward transmitted power. For example, the classic way of realizing high performance hybrids using quarter wavelength transmission lines is too large for power amplifier modules. This means that hybrids are realized using lumped components, i.e. using cross coupled resonators. As shown in FIG. 1, such a hybrid 100 has an input port 110, an output (or through or transmitted) port 120, a coupled (or auxiliary) port 130, and an isolated port 140 that is terminated by a resistor 145. The hybrid further includes first, second, third, fourth, fifth and sixth capacitors 161-166 and first and second cross-coupled inductors 171, 172, with each of the first, second, third and fourth capacitors connected between ground and a different one of the ports, with the first inductor 171 connected between the input port 110 and the output port 120, with the second inductor 172 connected between the coupled port 130 and the isolated port 140, with the fifth capacitor 165 connected between the input port 110 and the coupled port 130 and the sixth capacitor 166 connected between the output port 120 and the isolated port 140.
An illustrative embodiment of a prior art quadrature amplifier is depicted in FIG. 2. The amplifier comprises first and second amplifiers 210, 220, each having an input 212, 222 and an output 214, 224, a signal splitter 230, first and second impedance transformers 240, 250 connected to the outputs of the first and second amplifiers, and a signal combiner 260 connected to the outputs of the impedance transformers. Each amplifier 210. 220 may have one or more stages. Signal splitter 230 is a −3 dB hybrid that splits an input signal received at an input port 232 into two output signals of equal amplitude that are in phase quadrature (i.e., are 90 degrees apart in phase) at an output port 234 and a coupled port 236. One of these signals is conventionally referred to as the I signal and the other as the Q signal. One of the output signals from splitter 230 is applied to input 212 of amplifier 210 and the other output signal is applied to input 222 of amplifier 220.
The output impedance of each amplifier is on the order of 4 Ohms. The impedance matching transformers 240. 250 match this impedance to an impedance on the order of 50 Ohms. Signal combiner 260 is a −3 dB hybrid that combines the output signals at the outputs of the impedance matching networks 240, 250. Since the output signals are in quadrature, the signals are combined to produce an inphase signal at the output of the signal combiner. As is known in the art, a combiner can be made by using a splitter in reverse. Thus, combiner 260 receives the output signals from transformers 240, 250 at output port 264 and coupled port 266 and combines them to form an inphase signal at input port 262.
Implementation of quadrature power amplifiers such as those of FIG. 2 in modern communication equipment, however, is a challenge because of the physical size of the inductors required in the signal combiner.